In the operation of fast-neutron nuclear reactors, the cooling liquid of which generally consists of a liquid metal, such as sodium, it is necessary to carry out handling operations on fuel assemblies or mechanical components submerged in the liquid cooling metal of the reactor contained in the reactor vessel. It may also be necessary to conduct handling operations on fuel assemblies or mechanical components arranged in capacities filled with liquid sodium and associated with the reactor vessel. These handling operations can be necessary especially for ensuring the storage of irradiated fuel assemblies or the replacement of assemblies in the reactor core.
In a reactor, the upper level of the liquid metal is surmounted by a gaseous atmosphere generally consisting of an inert gas, liquid metals, such as sodium, being extremely reactive and liable to burst into flames spontaneously in contact with oxygen.
Outside the sector of nuclear reactors using a liquid metal coolant, it may be necessary to carry out handling operations alternating between a cold-gas zone and a zone containing a hot liquid, thus risking impairing the functioning of the grab.
There are known grabs of the mechanical type comprising gripping fingers which can be controlled remotely in order to carry out the pick-up or release of an object, such as a fuel assembly.
Such grabs can be designed to function when they are submerged in a liquid metal, such as sodium.
At least some of the components forming the grab, particularly its gripping members are constantly submerged in the liquid sodium and function at a temperature higher than the solidification temperature of this liquid metal.
Such mechanical grabs can, of course, also be designed to be used in a gaseous atmosphere. They are never submerged.
However, as regards operations for handling objects, such as fuel assemblies, submerged in a hot liquid metal, for example in order to extract these objects from the liquid metal and remove them into a zone occupied by a gaseous atmosphere at a temperature below the solidification temperature of the liquid metal, it is not possible to use these mechanical grabs because liquid metal is liable to settle and solidify on the movable members of the grab, at the moment when the grab passes from a position submerged in the liquid metal to an emerged position in which the grab is plunged into a gaseous atmosphere the temperature of which is below the solidification temperature of the liquid metal. The solidification of the liquid metal on the movable members of the grab can result in defective functioning of this grab, and especially in failure of the fingers of the grab to open and to release the object which is being handled.
To carry out handling operations on objects, such as fuel assemblies, submerged in a liquid metal, two different grabs are generally used, the handling operations taking place in two stages.
A first grab, functioning solely under hot conditions at a temperature above the solidification temperature of the liquid metal, makes it possible to pick-up of the head of the assembly below the level of the liquid metal and to lift the same head so as to place it in a emerged position emerged above the level of the liquid metal, and to release the head in a zone containing hot gas.
A second grab, functioning solely in a hot or cold gaseous atmosphere and therefore not being submersible, is then used in order to conduct a second handling operation which involves again picking up the head of the assembly previously taken out of the liquid metal. The second grab subsequently carries, out the lifting and handling of the fuel assembly in the gaseous atmosphere.
The disadvantage of this technique is that it requires the use of two different handling means during two successive phases of the handling operation.
There are known mechanical grabs consisting of a tubular body which is connected at one of its ends to a lifting means and in which are mounted gripping means, such as catching claws, movable in the radial direction of the tubular body between a position for picking up and a position for releasing the object which is being handled. A slide mounted slideably in the axial direction of the tubular body and connected to axial displacement means makes it possible to actuate the gripping means of the grab remotely.
However, such a device has never been designed so as to be capable of displacing an object between a position submerged in liquid metal and a position emerged in a hot or cold atmosphere.
The summary of the invention therefore, to provide a grab for lifting and displacing an object between a position submerged in a hot liquid, such as a molten metal, and a position emerged in a gaseous atmosphere, the grab comprising a body of tubular shape connected to a lifting means and a slide mounted slideably in the axial direction within the tubular body as well as means for gripping the object, which means are mounted within the tubular body and which are radially movable between a position for picking up and a position for releasing the object, the radial displacement of the gripping means being obtained as a result of the axial displacement of the slide within the tubular body, this grab being capable of functioning satisfactorily within the hot liquid and in a gaseous atmosphere during successive phases of the operation.
To this end, the tubular body of the grab according to the invention has a bell-shaped open end part, within which the slide is connected to the inner surface of the tubular body over its entire periphery by means of an axially deformable gas-tight elastic member, the gripping means being arranged within the bell-shaped end part of the tubular element between the elastic member ensuring its closure and its open end, so that, during the introduction of the lower part of the tubular body into the hot liquid in order to carry out the pickup and displacement of the object, gas is trapped inside the bell and forms a reserve, in which the gripping means are plunged during the pick-up and displacement of the object below the level of the hot liquid.